Data storage media are commonly used for storage and retrieval of data, and come in many forms, such as magnetic tape, magnetic disks, optical tape, optical disks, holographic disks or cards, and the like. Magnetic tape media remains an economical medium for storing large amounts of data. For example, magnetic tape cartridges or large spools of magnetic tape are often used to back-up data in large computing centers. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or notebook computers.
In magnetic media, data is typically stored as magnetic signals that are magnetically recorded on the medium surface. The data stored on the medium is typically organized along “data tracks,” and transducer heads are positioned relative to the data tracks to write data to the tracks or read data from the tracks. A typical magnetic storage medium, such as magnetic tape, typically includes several data tracks. Optical media, holographic media and other media formats can also make use of data tracks.
Servo patterns refer to signals or other recorded markings on the medium that are used for tracking purposes. In other words, servo patterns are recorded on the medium to provide reference points relative to the data tracks. A servo controller interprets detected servo patterns and generates position error signals. The position error signals are used to adjust the lateral distance of the transducer head relative to the data tracks so that the transducer head is properly positioned along the data tracks for effective reading and/or writing of the data.
With some data storage media, such as magnetic tape, the servo patterns are stored in specialized tracks on the medium, called “servo tracks.” Servo tracks serve as references for the servo controller. Servo tracks typically hold no data except for information that is useful to the servo controller to identify positioning of a transducer head relative to the surface of the medium.
The servo patterns recorded in the servo tracks may be sensed by one or more servo heads. For example, servo heads may be dedicated heads that read only servo patterns in the servo tracks. Alternatively, servo heads may be integrated with a read/write head. In any case, once a particular servo track is located by the servo head, a data track can be located on the medium according to the data track's displacement from the servo track. The servo controller receives detected servo signals from the servo heads and generates position error signals, which are used to adjust positioning of a read/write head relative to the data tracks.
Servo patterns are referred to as pre-recorded when they are recorded during the fabrication of the media. In other words, pre-recorded servo patterns are servo patterns recorded in the media prior to the media being used for storage of data. These pre-recorded servo patterns allow the media to achieve higher storage densities because the servo patterns enable positions on the media to be located with greater precision. In particular, servo patterns facilitate the ability to pinpoint locations on media surfaces with greater accuracy. Therefore, servo patterns allow for smaller amounts of media surface to be used to store units of data.
Amplitude-based servo patterns refer to servo patterns in which detection of the servo signal amplitude enables identification of head positioning relative to the medium. In other words, as the head passes relative to the medium, signal amplitudes of detected servo patterns can be used to determine whether the head is positioned correctly relative to a track on the medium. Amplitude-based servo patterns are commonly implemented in magnetic tape media, but may also be useful in other media.